In a medium bowl, whisk together olive oil, lemon juice, garlic, oregano, salt, and pepper until emulsified.
Incorporate chicken cubes into the marinade, ensuring all surfaces are coated.
Heat a heavy-bottomed skillet or grill pan over medium-high heat.
Cook chicken for 8 to 10 minutes, turning occasionally, until the internal temperature reaches 165 degrees Fahrenheit.
Divide the 2 cups of cooked quinoa equally into four serving bowls.
Arrange the cooked chicken, tomatoes, cucumber, red onion, and olives in distinct sections over the quinoa.
Distribute the crumbled feta cheese evenly across the bowls and garnish each with a 2-tablespoon portion of tzatziki sauce.